Field of the Invention and Related Art Statement
The present invention relates to an ultrasonic probe for use in an ultrasonic endoscope.
Similar to ultrasonic probes used in various fields of diagnoses, the ultrasonic probe for use in the ultrasonic endoscope can inspect the construction of a body under inspection by applying an electric signal to electrodes applied on opposite surfaces of a piezo-electric element to emit an ultrasonic wave toward the body, receiving an ultrasonic wave reflected by the body, converting the received wave into an electric echo signal, and processing the electric echo signal to display an ultrasonic image of the body. The ultrasonic probe for use in the endoscope has to be constructed extremely small in size, because the ultrasonic probe is arranged in a distal end of a very thin insertion section of the endoscope which is insertable into a cavity of the body.
FIG. 1 is a cross sectional view showing the construction of a known ultrasonic probe for use in an ultrasonic endoscope. The ultrasonic probe comprises a piezo-electric element 1 made of piezo-electric material such as PZT. On opposite surfaces of the piezo-electric element 1 are applied a front surface electrode 2 and a rear surface electrode 3, respectively. By applying an electric signal, particularly a pulse signal across these electrodes 2 and 3, the piezo-electric element 1 is vibrated to emit an ultrasonic wave. On the front surface electrode 2 there is provided a lens layer 4 for converging the ultrasonic wave emitted by the piezo-electric element 1.
On the rear surface electrode 3 there is provided a damping layer 5 for absorbing an undesired ultrasonic wave emitted from the rear surface electrode 3. The damping layer 5 may be made of a synthetic resin such as epoxy resin having conductive material particles such as tungsten powder embedded therein, so that the damping layer is generally electrically conductive. The piezo-electric element 1, lens layer 4 and damping layer 5 are mounted in a housing 6 made of, for instance metal by means of an insulating layer 7 in the form of a sleeve. A core wire 9a of a signal cable 9 for supplying the electric signal to the piezo-electric element 1 is connected to the rear surface electrode 3 and a shielding conductor 9b of the cable 9 is connected to the front surface electrode 2 by soldering the shielding conductor to the housing 6 and by connecting the housing to the front surface electrode 2 by means of a conductive wire 10. Since the damping layer 5 has the electrical conductivity, it is required to prevent the short-circuiting between the front and rear surface electrodes 2 and 3 via the damping layer. To this end, on a rear surface of the damping layer 5 is provided an insulating layer 8.
FIGS. 2 and 3 illustrate the known ultrasonic endoscope having the known ultrasonic probe. FIG. 2 shows the construction of a distal end of the insertion section of the endoscope. Inside a cap 11 there is arranged an ultrasonic probe 12 which emits an ultrasonic wave 13. A reference numeral 14 denotes a light guide for illuminating the body under inspection, 15 represents an objective lens for forming an optical image of the body and 16 shows an opening of a forceps channel through which various kinds of forceps can be inserted. FIG. 3 is a schematic view illustrating a portion of the endoscope, in which a portion A represents an insertion section which is insertable into the cavity of the body and a portion B denotes the remaining portion, i.e. a portion outside the body.
In the ultrasonic probe shown in FIG. 1, since the lens layer 4 and damping layer 5 are directly applied on the piezo-electric element 1, the electrostatic capacitance of the element 1 is equivalently increased and thus the vibrating efficiency of the element is liable to be decreased. It has been known to increase the vibrating efficiency of the element 1 by providing a matching circuit 17 including a matching coil 17a as illustrated in FIG. 3. In the ultrasonic endoscope, since it is necessary to make the ultrasonic probe as small as possible, the matching coil 17a has to be provided in the portion B as shown in FIG. 3.
On the other hand in order to obtain the ultrasonic image having the high resolution, it is required to increase the frequency of the ultrasonic wave. In this case, the matching coil 17a is arranged at the proximal end of the cable 9, the electrostatic capacitance and ohmic resistance of the relatively long cable 9 could be no more ignored, so that the resonance point of the ultrasonic vibrating element including the piezo-electric element 1 and surface electrodes 2 and 3 might be shifted from a designed resonance point and the sufficient matching could not be attained. In this manner, the ultrasonic vibrating element could not be vibrated at a desired high frequency. It should be noted that the above mentioned capacitance and ohmic resistance of the signal cable do not affect seriously the function of the element 1 when the vibrating frequency is rather low such as 1 to 7.5 MHz.